A Modular Multilevel power Converter (MMC), also known as Chain-Link Converter (CLC), comprises a plurality of converter cells, or converter sub-modules, serially connected in converter branches, or phase legs, that in turn may be arranged in a star, delta, direct or indirect converter topology. Each converter cell comprises, in the form of a half-bridge or full-bridge circuit, a capacitor for storing energy and power semiconductor switches such as insulated gate bipolar transistor (IGBT) devices, gate-turn-off thyristor (GTO) devices, integrated gate commutated thyristor (IGCT) devices, or Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) devices for connecting the capacitor to the converter branch with one or two polarities. MMCs may be used in electric power transmission systems such as Static Synchronous Compensator (STATCOM), Frequency Converters in direct or indirect topology and High-Voltage Direct Current (HVDC) transmission.
For control and protection reasons, phase leg current measurements are mandatory in a converter. FIG. 1 illustrates a typical MMC in delta topology in which the phase leg between points 1 and 2 has a current i12 which is measured by a sensor (transducer) connected between the phase leg and a grounded input/output (I/O) board of a central control unit of the converter. If the converter is a high-voltage converter, a lot of insulation is needed for the sensor since the voltage difference between the phase and ground is high. The high-voltage converter may be located in a dedicated room or building, and the sensor and its insulation may be located in a wall bushing into said room or building.
Control functions can include:                Current control        Cell sorting and/or selection algorithms        
Protection functions can include:                Overcurrent protection        Semiconductor thermal model        
Since MMC converters can be designed for high voltages (several 10 kV:s, e.g. U12=36 kVrms), current measurements can be expensive, or there may be no sensors available on the market due to the high insulation level. Insulation of the current transducer has to be designed for the voltage U0 of FIG. 1, which is in normal operation approximately U12/sqrt(3) or in failure cases (earth fault in the 3-phase system) even approximately U12.
An alternative, to reduce the insulation level, is to place a current sensor in each individual cell as an integrated part of the cell circuitry. The sensor may then be connected between a conducting line in each cell of the phase and the cell controller of the cell, each at high voltage potential, reducing the voltage difference which has to be handled by the insulation. However, since each cell of the phase leg has to know the present current, each cell has to be provided with its own current measurement sensor, which increases the cost of the converter.
US 2014/0210431 discloses a multi-cell power supply for receiving power from a source and delivering power at an output terminal to a load. The multi-cell power supply includes a first power cell coupled to the source, and a first current sensor circuit. The first power cell provides a first output current, and includes a first output terminal coupled to a reference node of the multi-cell power supply, and a second output terminal coupled to the output terminal. The first current sensor circuit includes a first current sensor and a power supply. The first current sensor is coupled to the first output terminal of the first power cell, and measures the first output current. The power supply is coupled to either the reference node or a floating ground node of the first power cell, and provides power to the first current sensor.
Thus, the current sensor circuit is outside the power cell but connected to the power cell to measure its output current. The current sensor circuit, as well as the power cell, may each have a fiber optic interface for communication with a central controller, whereby information about the measured current may be forwarded from the current sensor circuit to the power cell via the central controller.